Polyimides have long been used as gas separation membranes. Generally, however, this class of polymers exhibits an inverse relationship between the gas permeation rate (flux) and the selectivity of one gas over other gases in a multi-component gas mixture. Because of this, prior art polyimide gas separation membranes tend to exhibit either high flux at the sacrifice of gas selectivities or high gas selectivities at the sacrifice of flux.
For example, U.S. Pat. Nos. 4,705,540 and 4,717,394 teach that certain alkyl-substituted polyimide material membranes have ultrahigh flux but suffer from only moderate gas selectivities.
The polyetherimide gas separation membranes taught in U.S. Pat. No. 4,156,597, exhibit productivities of about 191 centiBarrers when separating oxygen from air and oxygen/nitrogen selectivity of 5.3.
Polyimide gas separation membranes taught in U.S. Pat. No. 4,378,400, have, at an oxygen/nitrogen selectivity of 7.3, a productivity of only 46 centiBarrers.
The polyimide membranes taught in U.S. Re. Pat. No. 30,351, U.S. Pat. Nos. 3,822,202, and 3,899,309 also suffer from moderate gas selectivities.
U.S. Pat. No. 4,746,474 teaches a process for making free-standing, pinhole-free, ultrathin polyimide films, including those incorporating phenylindane residues in a polyimide backbone chain, by casting the polymer on water. The reference alludes to use of the films in gas separation and in end uses where controlled release of drugs is needed.
It would be highly desirable for polyimide gas separation materials to exhibit high gas selectivities while maintaining high fluxes.